/in/foo.cc: In function 'Result Match(std::__cxx11::string&)':
/in/foo.cc:17:20: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
  for (int i = 0; i < s.length(); i++)
                  ~~^~~~~~~~~~~~
/in/foo.cc:21:6: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]
   if (s[i] == ')')
      ^
/in/foo.cc:16:6: warning: unused variable 'cnt' [-Wunused-variable]
  int cnt = 0;
      ^~~
/in/foo.cc: In function 'int main()':
/in/foo.cc:45:16: warning: format '%s' expects argument of type 'char*', but argument 2 has type 'std::__cxx11::string* {aka std::__cxx11::basic_string<char>*}' [-Wformat=]
  scanf("%s", &s);
                ^